Immunology

Full details for "Interferon-Gamma Release Assay (IGRA)".

Name

Interferon-Gamma Release Assay (IGRA)

Alternative name/Profile

Latent Tuberculosis (TB), Mycobacterium, QuantiFERON-TB Gold, IFN-Gamma, Purified Protein Derivative (PPD)

Department

Immunology

Investigation

Enzyme Immunoassay (ELISA) (Manual, QuantaFERON-TB Gold)

Specimen type

Blood

Sample type

Serum

Sample container & volume

Contact Laboratory

Frequency of analysis

Batch basis

Turnaround time

21 days

Availability

Monday to Thursday, 9 to 5

Notes

Useful for:

Investigating patients for latent Mycobacterium Tuberculosis (TB) infection or active Mycobacterium Tuberculosis (TB) infection.

Clinical Information:

Latent Tuberculosis is an asymptomatic condition that may progress to active Tuberculosis in some individuals.
The Interferon-Gamma Release Assay (IGRA) measures the level of the cytokine interferon-gamma (IFN-gamma).
Interferon-gamma (IFN-gamma) is released by lymphocytes in a cell-mediated immune responce to mycobacterial proteins.
These proteins include ESAT-6, CFP-10 and TB7.7, and are absent from all BCG strains and most non-tuberculous mycobacteria.

Please Note:

The Interferon-Gamma Release Assay (IGRA) requires specialised blood collection tubes. These tubes are available for collection in the laboratory.
Correct handling of the blood collection tubes is essential.
Blood should be collected into the Nil tube (grey) first, followed by the mitogen tube (purple) and finally into the TB tube 1 and 2.

Tubes should be between 17° to 25°C at the time of blood filling.

The black mark on the side of the tubes indicates the validated range of 0.8–1.2 ml. If the level of blood in any tube is outside this range a new blood sample should be obtained.

If a “butterfly needle” is being used to collect blood, a “purge” tube should be used to ensure that the tubing is filled with blood prior to the QFT-Plus tubes being used.

Immediately after filling the tubes, invert the samples ten times to ensure the entire inner surface of the tube is coated with blood.
Collected samples are only accepted into the laboratory between the hours of 9 to 5, Monday to Thursday.
Although ESAT-6, CFP-10 and TB7.7 are absent from all BCG strains and most non-tuberculous mycobacteria, patients infected with M. kansasii, M. szulgai or M. marinum may give a false positive Interferon-Gamma Release Assay (IGRA) result.
A negaitve Interferon-Gamma Release Assay (IGRA) result does not preclude the possibility of M. tuberculosis infection. False negatives can be due to incorrect handling of the blood collection tubes, the stage of the infection or co-morbid infections which affect immune function.
All positive results should be followed by further medical evaluation.
If the result is indeterminate for TB antigen responsiveness, this may be related to a wide variety of factors including immunosuppressant medication. A repeat sample will be requested by our laboratory. A subsequent second indeterminate result may benefit from discussion with the clinical immunology or TB teams

Related links

Reference range

Nil (IU/ml)	TB Antigen minus Nil (IU/ml)	Mitogen minus Nil (IU/ml)	QFT result	Report/ Interpreation
≤0.8	<0.35	≥0.5	Negative	M. Tuberculosis infection NOT likely
	≥0.35 and <25% of Nil value	≥0.5	Negative	M. Tuberculosis infection NOT likely
	≥0.35 and ≥25% of Nil value	Any	Positive	M. Tuberculosis infection likely
	<0.35	<0.5	Intermediate	Results are indeterminate for TB-Antigen Responsiveness
	≥0.35 and <25% of Nil value	<0.5	Intermediate	Results are indeterminate for TB-Antigen Responsiveness
>8.0	Any	Any	Intermediate	Results are indeterminate for TB-Antigen Responsiveness